The present invention relates in general to slide bearing surfaces and bearing elements and in particular, to a new and useful slide bearing surface element which can be used in mechanical bearings and particularly in prosthetic joint replacements.
It is known to replace joints in the body, for example a hip joint, with artificial or prosthetic joints. An example is to replace the head of the femur or large leg bone with a metal ball made of stainless steel or vitallium. The socket in the pelvis or acetabulum is enlarged and a cup is inserted and secured to the pelvis for receiving the metal ball. Such joints are long lasting, however, they do not reproduce the resiliency and thermal characteristics of the living hip joint and are therefore, not entirely acceptable.
Some examples of prior art approaches to joint replacements can be found in U.S. Pat. No. 4,123,806 to Amstutz et al. Here it is disclosed that the cup can be made of ultra high molecular weight polyethylene and that a thin metal shell can be used rather than a total metal ball replacement, over the femoral head. Other examples of the prior art can be found in U.S. Pat. No. 3,829,904 to Ling et al, U.S. Pat. No. 3,723,995 to Baumann, U.S. Pat. No. 3,924,275 to Heinke et al, U.S. Pat. No. 3,893,196 to Hochman and U.S. Pat. No. 4,195,368 to Patrichi.
As already mentioned, many of the currently available artificial hip joints are made of a metal femoral replacement stem articulating within an ultra high molecular weight polyethylene, UHMWPE, cup. Both elements are secured with acrylic bone cement. In addition to the lack of resiliency noted above, these hip replacements suffer from additional problems including bone resorption, acrylic bone cement failure, and a loss or fatigue of the metal stem. This has led to so called cup or shell arthroplasty. In this technique only a layer of bone is removed from the femoral head and from the interior of the acetabular cup. The removed bone is replaced by matching cups and shells, the shell fastened over the remaining femoral head and the cup fastened into the enlarged acetubulum. Since the shell is made of metal however, problems remain due to a mismatching of elasticity and other properties between the metal and the living bone which it replaces. Young's modulus of bone is from about 5.times.10.sup.4 psi to about 3.times.10.sup.6 psi. The Young's modulus for steel is about 30.times.10.sup.6 psi. While replacing the metal shell with a plastic shell might advantageously change the elasticity to be closer to that of bone, it is known in the art that plastic bearing parts can only be used in light loading applications, for example, of about 200 psi. For use particularly as a hip joint replacement, the joint must, however, be capable of withstanding 500 to 1,000 psi and, to be servicable for a relatively large number of years.
These considerations are also applicable to bearing or sliding elements such as gears, vanes, pistons and the like in mechanical applications other than prosthetic devices.